Field of the Invention
The invention relates to circuits which safeguard circuits against overload currents. In particular, the invention relates to an overload protection circuit for line drivers.
In order that digital signals are transmitted in an uncorrupted manner from one integrated circuit to a further integrated circuit, it must be ensured that the connecting lines do not impair the signals. The influence of the connecting lines cannot be disregarded if, in the case of steep signal edges, the propagation time through the connecting lines is at least of the order of magnitude of the rise time of the circuit. A simple connecting wire between the circuits is no longer sufficient in this case. In order to avoid serious signal deformations, lines with a defined characteristic impedance are used, which are terminated with a characteristic impedance. Two insulated wires twisted together are often used for this purpose. Such lines are called twisted-pair lines. A twisted-pair line with approximately 100 turns per meter has a characteristic impedance of approximately 100 xcexa9. Owing to the low terminating resistance, the transmitter must supply a correspondingly high output current. In order to satisfy this requirement, line drivers are used as the transmitters or drivers.
For interference-immune signal transmission, line drivers are often configured in such a way that they feed the two wires of the twisted-pair lines with symmetrical, complementary signals. A comparator is used as a receiver. In this operating mode, the information is determined by the polarity of the differential voltage and not by the absolute value of the level. In this case, an interference pulse merely brings about a common-mode modulation, which remains ineffective owing to the difference formation in the comparator.
A known line driver has differential amplifiers. The inverting inputs of the differential amplifiers are fed by input currents. The input currents contain the information to be communicated. The input currents are identical in terms of their temporal characteristics and the magnitude of their amplitudes; they differ merely through different signs. The non-inverting inputs of the differential amplifiers are connected to a constant-voltage source. The differential amplifiers are connected up as current-voltage converters with feedback resisters. The outputs of the differential amplifiers feed the primary-side inputs of a transformer. The transformer communicates the signals to a twisted-pair line.
The line driver can be connected up to a load on the output side. The load stands as an equivalent circuit diagram for the transformer and the twisted-pair line. The load is characterized by a load resistance.
In the case of the line driver, a short circuit of the outputs with the supply voltage or earth can lead to static overload currents which, under certain circumstances, entail destruction of the line driver. In the case of an excessively small load resistance, with pulsed complementary input currents, an increased pulsed current flow through the line driver can occur.
In previous line drivers, protection against an overload and/or short circuit is essentially tackled with two different protection circuits. A first conventional protection circuit is based on a resistor connected in series between the source terminal of the driver transistor and the supply voltage. The voltage drop across the resistor is compared with a reference voltage by a read-out amplifier. The protection circuit has the disadvantage that the output dynamic range of the line driver is limited by the additional voltage drop.
Furthermore, an additional operational amplifier is required as the read-out amplifier, and must be tuned, since the offset voltage of the operational amplifier is present serially and hence additively with respect to the voltage dropped across the resistor. A further disadvantage of the protection circuit can occur with pulsed input currents. If the bandwidth of the read-out amplifier is not significantly larger than the frequency of the pulse train of the input currents that feed the line driver, a pulsed overload is not identified.
A second conventional protection circuit for line drivers provides for the output current flowing through an output transistor of the line driver to be limited by a clamping transistor. To that end, the output current is mirrored into the drain-source path of a further transistor by a current mirror. The clamping transistor is connected up in such a way that its gate voltage is a function of the mirrored output current. In the case of a gate voltage above the threshold voltage, the clamping transistor turns on and limits the gate voltage of the output transistor. As a result, the maximum output current flowing through the drain-source path of the output transistor is limited. The protection circuit has the disadvantage that its switching threshold is greatly dependent on the threshold voltage tolerances of the clamping transistor. Moreover, jitter about the changeover point leads to unreliable detection of an overload current. Furthermore, the protection circuit is subject to great dependencies on the temperature and on tolerances of the production process.
It is accordingly an object of the invention to provide an overload protection circuit for line drivers that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which precisely detects the occurrence of overload currents above a predetermined detection threshold and whose detection threshold is adjustable in a simple manner. In particular, the intention is to provide a circuit for protecting line drivers against overload currents which contains the abovementioned detection circuit and which protects the line driver even against overload currents that occur in pulsed fashion and recur at short time intervals.
With the foregoing and other objects in view there is provided, in accordance with the invention, a detection circuit for ascertaining a presence of an overshooting of a current flowing through a line above a predetermined value. The detection circuit contains a first current mirror having a first input transistor, a first output transistor, an input, and an output outputting an output signal. A second current mirror is provided and has a second input transistor, a second output transistor, an input and an output outputting an output signal. The first input transistor has a first threshold voltage being greater than a saturation voltage of the second output transistor. The second input transistor has a second threshold voltage being greater than a saturation voltage of the first output transistor. The input of the first current mirror and the output of the second current mirror are connected to each other at a common first node and the common first node is present at an end of the line. The input of the second current mirror and the output of the first current mirror are connected to each other at a common second node. A first controllable current source is provided and has an output connected to the common first node and outputs a first current. A second controllable current source is provided and has an output connected to the common second node and outputs a second current. The first current and the second current have a linear relationship to one another, and the predetermined value is a function of the linear relationship of the first current and second current. An evaluation unit is coupled to the first current mirror and to the second current mirror. The evaluation unit ascertains the presence of the overshooting of the current flowing through the line above the predetermined value from an evaluation signal corresponding to the output signal of one of the first current mirror and the second current mirror.
The first and second input and output transistors may be configured either as MOS transistors or as bipolar transistors. An essential concept of the invention is that the input of the first current mirror and the output of the second current mirror have a common first node. Equally, the input of the second current mirror and the output of the first current mirror are connected to one another via the common second node. Furthermore, the first node is present at an end of the line. The first current can be coupled out at the output of the first controllable current source and the second current can be coupled out at the output of the second controllable current source. The first and second controllable current sources are integrated into the detection circuit in such a way that the first current feeds the first node and the second current feeds the second node. Both controllable current sources may be, for example, either voltage-controlled or current-controlled. Furthermore, the two controllable current sources necessitate one another, so is that the first current and the second current have a linear relationship to one another. By way of example, the two currents may be proportional to one another, i.e. the quotient of the first and second currents is always constant. The predetermined value, the overshooting of which causes an overload current to be indicated by the detection circuit, is a function of the linear relationship of the first and second currents. As a result, the predetermined value can be set by way of the first and second controllable current sources. The evaluation unit of the detection circuit uses an evaluation signal, which corresponds to an output signal of the first or second current mirror, to identify whether a current greater than the predetermined current flows through the line. The output signal of the first and of the second current mirror may be the current supplied by the first and second current mirror, respectively, or the voltage present at the output of the first and second current mirror respectively.
One advantage of the detection circuit according to the invention is that the detection threshold, the overshooting of which causes an overload current to be indicated by the detection circuit, is a function of the currents supplied by the controllable current sources. Controllable current sources can be produced with a high quality factor and the current supplied by them can be set to a very precise value. Therefore, in comparison with previous detection circuits, the predetermined value can be set simply and precisely. The predetermined value may possibly even be set by software. Furthermore, through suitable dimensioning of the first and second input and output transistors and of the first and second controllable current sources, it is possible to obtain a high bandwidth of the detection circuit, so that even momentary pulsed instances of overshooting of the predetermined current value can be ascertained. A further advantage of the detection circuit according to the invention over conventional detection circuits is its simple topology. Evaluation amplifiers in the form of complex operational amplifiers, as are used in many detection circuits in the prior art can be completely obviated. Moreover, the present detection circuit has a small dependence relative to temperature changes and tolerances of the production process. One reason for this is that the first and second current mirrors are interconnected in crossed fashion, as a result of which the temperature dependencies of the two current mirrors cancel one another out. In addition, the crossed interconnection of the two current mirrors allows the detection threshold to be provided with a hysteresis, so that jitter can be suppressed. Moreover, the crossed interconnection of the two current mirrors enables precise detection of overload currents.
The controllable current sources are advantageously configured as current mirrors. To that end, the first controllable current source has a third current mirror, which is characterized by a third output transistor and a third input transistor. The third input transistor is fed by a constant-current source. The second controllable current source contains a fourth current mirror having a fourth output transistor. The input transistor of the fourth current mirror is again the third input transistor. In this advantageous configuration of the detection circuit according to the invention, the predetermined value is a function of the current supplied by the constant-current source. The configuration of the controllable current sources as current mirrors is one possible, simple realization of the controllable current sources. In this case, the predetermined value can be established particularly simply through the dimensioning of the third and fourth current mirrors and through the current supplied by the constant-current source. The transistors of the third and fourth current mirrors may again be configured either as MOS transistors or as bipolar transistors.
In accordance with a further advantageous configuration of the invention, the second input transistor is connected to a read-out transistor to form a read-out current mirror, the second input transistor also being the input transistor of the read-out current mirror. On account of the current-mirror-like interconnection, an output signal of the read-out current mirror can be used as the evaluation signal. The output signal of the read-out current mirror may be either the output current or the output voltage of the read-out current mirror. The advantage of the configuration is the indirect measurement of the output signal of the second current mirror. As a result, the crossed-interconnected first and second current mirrors are not loaded by the evaluation unit or impaired in their functioning.
It is advantageous for a third controllable current source to feed the output of the read-out current mirror. The third controllable current source may be configured, for example, as a fifth current mirror that has a fifth output transistor and whose input transistor is the third input transistor.
As an alternative to the realization of the first and second controllable current sources as current mirrors, it may advantageously be provided that each controllable current source contains a MOS transistor whose gate potential is driven by a regulating circuit. To that end, the first controllable current source has an operational amplifier, a third output transistor that is a MOS transistor, and a constant-voltage source. A first terminal of the drain-source path of the third output transistor is connected to the first node and the gate potential of the third output transistor is driven by the operational amplifier in such a way that the voltage provided by the constant-voltage source is present at the second terminal of the drain-source path of the third output transistor. The second controllable current source contains a fourth output transistor, which is likewise a MOS transistor and whose first terminal of its drain-source path is connected to the second node. The gate terminal of the fourth output transistor is connected to the gate terminal of the third output transistor and is thus likewise driven by the operational amplifier. In this configuration, the predetermined value is determined by the voltage output by the constant-voltage source. Consequently, the configuration of the controllable current sources is voltage-controlled, in contrast to the current-controlled configuration described above.
It is advantageous that the non-inverting input of the operational amplifier may be connected to the output of the constant-voltage source and the inverting input of the operational amplifier may be connected to the second terminal of the drain-source path of the third output transistor. Furthermore, it is advantageous if the output of the operational amplifier is connected to the gate terminals of the third and fourth output transistors. The essential advantage of the voltage-controlled configuration of the first and second controllable current sources is that the accuracy and the synchronism of the first and second currents supplied by the controllable current sources are very high. In this respect, the configuration according to the invention even surpasses the current-controlled configuration described above.
In the case of the voltage-controlled realization of the controllable current sources, in the same way as in the case of the current-controlled realization, it may advantageously be provided that the second input transistor is connected to a read-out transistor to form a read-out current mirror, the second input transistor also being the input transistor of the read-out current mirror. On account of the current-mirror-like interconnection, an output signal of the read-out current mirror can again be used as the evaluation signal. The advantage of the configuration again resides in the indirect measurement of the output signal of the second current mirror. As a result, the crossed-interconnected first and second current mirrors are not loaded by the evaluation unit or impaired in their functioning.
A further advantageous configuration of the invention provides for the first and second input and output transistors and also the read-out transistor to have the same conductivity type and to be, in particular n-channel MOSFETs. The use of transistors with the same conductivity type for the crossed-interconnected current mirrors increases the accuracy of the detection threshold of the detection circuit and makes the detection circuit less dependent relative to tolerances of its production process. Furthermore, the use of MOS transistors has the advantage that the current flowing through their drain-source paths can be precisely interrupted by suitable choice of the gate potential. On account of the function of the first and second input and output transistors, this ensures a precise detection of overload currents by the detection circuit according to the invention.
A protection circuit according to the invention, whose task is to limit a signal potential present on a signal line below a predetermined signal potential value, contains a converter circuit, a detection circuit and a protection device. The converter circuit serves for converting the signal potential into a corresponding signal current. By use of the detection circuit, it is possible to ascertain whether a current flowing through a line exceeds a predetermined value. In this case, the current corresponds to the signal current. The detection circuit can have the same features as the detection circuit described above. The protection device serves for limiting the signal potential below the predetermined signal potential value, if an overshooting of the current flowing through the line above the predetermined value is indicated by the detection circuit. The overshooting of the current flowing through the line above the predetermined value corresponds to an overshooting of the signal potential above the predetermined signal potential value.
The protection circuit according to the invention can also be configured in such a way that it limits a signal current flowing through a signal line below a predetermined signal current value. It then has the same features as the above-described protection circuit for limiting the signal potential present on the signal line, except for the fact that the converter circuit is omitted. The converter circuit can be obviated since a signal current is already involved in this configuration.
It is advantageous that the protection device of the protection circuit for limiting a signal current flowing through a signal line have a MOS transistor through whose drain-source path the signal line leads. Furthermore, the protection device contains a clamping transistor that controls the gate potential of the MOS transistor. In the event of an overshooting of the current flowing through the line above the predetermined value, the clamping transistor limits the signal current below the predetermined signal current value. It may also be provided that the clamping transistor in this case completely turns off the MOS transistor for the signal current.
Since the protection circuit according to the invention contains the above-described detection circuit according to the invention, the protection circuit has the same advantages over conventional protection circuits as the detection circuit.
A line driver configuration according to the invention contains a differential line driver and four protection circuits, whose features in each case correspond to the protection circuit described above. The line driver configuration has a differential signal input with a first and second input terminal and a differential signal output with a first and a second output terminal. The signal input and the signal output of the line driver configuration are simultaneously the signal input and the signal output, respectively, of the line driver. A load can be connected between the output terminals. A signal is transmitted to the load by the line driver. Furthermore, the line driver contains a first MOS transistor, through whose drain-source path a first signal line leads, through which a first signal current flows. Analogously, the line driver has a second, third and fourth MOS transistor, through whose drain-source path a second, third and fourth signal line, respectively, leads, through which a second, third and fourth signal current, respectively, flows. The first and second signal currents feed the first output terminal, and the third and fourth signal currents feed the second output terminal of the line driver configuration. Each of the four protection circuits limits one of the four signal currents below the predetermined signal current value. In this case, each of the four MOS transistors of the line driver configuration is identical to a MOS transistor of the protection device of the four protection circuits.
The line driver configuration according to the invention contains the above-described protection circuit according to the invention. The line driver configuration thus has the same advantageous over conventional line driver configurations and line drivers as the protection circuit.
In accordance with one advantageous configuration of the line driver configuration according to the invention, in the event of an overshooting of one of the four signal currents above the predetermined signal current value, all four signal currents are limited by the protection circuits. This prevents the situation in which, in the event of the overshooting of only one signal current and the subsequent limiting of the signal current, the overload current flows away via other signal lines.
A further advantageous configuration of the line driver configuration according to the invention provides for the line driver configuration to have a counter, which may be, in particular, a digital counter. The counter counts successive instances of overshooting of the signal currents above the predetermined signal current value and limits the signal currents if the number of instances of overshooting within a predetermined period of time exceeds a predetermined number. This configuration makes it possible to detect and limit overload currents that occur in pulsed fashion and recur at short time intervals. This prevents impairment or even destruction of the line driver configuration in the event of an excessively small resistive load. Furthermore, this configuration prevents the line driver from being inhibited in the event of a pulsed overshooting of the signal current that occurs once. Such a one-off pulsed overshooting need not necessarily be caused by an excessively small resistive load, but rather may have other reasons. In order to maintain the function of the line driver in the case of such an event, the counter acts as a filter that limits the signal currents only after a predetermined number of instances of overshooting within a predetermined period of time.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an overload protection circuit for line drivers, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.